1. Field of the Invention
This invention relates to electromechanical devices for sound generation, and particularly to high-sounding horns for use in motor vehicles.
2. Description of Related Art
Sound generating devices of the electromagnetic excitation type currently consist of:
a resilient steel diaphragm carrying in its centre the mobile part (armature) of an electromagnet; PA1 an electric switch with a normally closed contact connected in series with the power feed to the electromagnet; PA1 an adjustment screw which determines the switch contact opening and PA1 a diffuser which resonates at the same frequency as the metal diaphragm. PA1 As the sound output of the horn depends on the time at which the switch operates, it is difficult to obtain maximum sound output because of the difficulty of fixing or adjusting the switch operation point. PA1 The sound output is subject to considerable fall-off with time due to the mechanical instability of the switch operation points. PA1 The switch contacts are subject to sparking which causes them to wear and lead to a variation in their time of operation, with reduction in sound output. PA1 The contact sparking creates electromagnetic waves which can be troublesome to the electronic systems increasingly used in modern motor vehicles. PA1 the need for an oscillator the frequency of which is stable with varying feed voltage and having a frequency-temperature characteristic curve equal to that of the mechanical unit; and PA1 in order to limit to a minimum any differences between the oscillator frequency and the diaphragm resonance frequency, the diaphragm production tolerances must be restricted or alternatively a selection and coupling procedure must be implemented.
When the electromagnet is electrically powered, it attracts the armature rigid with the resilient diaphragm. When the diaphragm has nearly attained its maximum travel, the switch connected in series with the electromagnet coil is opened by a push rod operated by the mobile assembly of the electromagnet. At this point the elastic energy accumulated by the diaphragm is restituted by reaction with the fixed structure to which it is connected, so that the diaphragm reverses its direction of movement. In this manner it again closes the switch which, again exciting the electromagnet, causes the diaphragm to commence a new oscillation cycle at a frequency equal to the resonance frequency of the electromechanical system.
These normal switch devices have considerable drawbacks, which can be summarized as follows:
To obviate these drawbacks, different methods have been conceived for controlling the excitation of the electromagnet coupled to the resilient steel diaphragm, these still being essential elements for the low-cost generation of high-intensity sound at frequencies less than one kilohertz.
The first alternative to the switch uses electronic oscillators operating at a vibration frequency approximately equal to the resonance frequency of the electromagnetic system; with this method the oscillator output controls an electronic switch connected in series with the coil, thus replacing the mechanically operated switch.
However, this method has certain drawbacks which can be summarized as follows:
All this results in high production costs which are difficult to accept by the user.
The aforesaid drawbacks can be obviated by linking the electronic oscillator frequency to the resonance frequency of the resonance frequency of the electromechanical unit which generates the sound. Such a method has already been proposed in French patent 1,428,483, which is now in public domain.
FIG. 1 shows the schematic diagram of said patent. In this figure a transducer S sensitive to diaphragm vibration is coupled to the diaphragm M of a horn X. The transducer S can be a known sensor sensitive to the vibration of the resilient diaphragm M of the horn, to generate at its output a voltage signal having a frequency corresponding to the vibration frequency. The transducer S feeds its signal to the input of an amplifier .mu. via a positive feedback circuit .phi., it being thus suitably amplified and then fed to the electromagnet E. The resultant vibration of the diaphragm M results in the reproduction of a voltage signal in the sensor S greater than that which it had generated but of coincident phase and frequency. The required oscillator with a resonance frequency the same as that of the electromagnetic sound generation system is therefore obtained.
A horn using an electronic circuit based on the above principle has better characteristics than a horn incorporating a mechanical switch or a fixed frequency electronic circuit, however the characteristics are insufficient for a high-sounding horn. To improve the sound output in relation to the current absorbed by the electromagnet in horns with a mechanical switch or fixed frequency electronic circuit it is already known to use an arrangement which exploits to a maximum the greater force of attraction which the electromagnet exerts on the armature when the air gap is reduced to the allowable minimum.
This arrangement consists of prolonging the electrical feed to the electromagnet beyond 50% of the inherent frequency period of the electromechanical system. The mean optimum value of the feed:response ratio is 65%:35%. It therefore follows that by applying this electromagnet feed concept the diaphragm oscillation is no longer sinusoidal. A sized spacer can be provided for each horn positioned along the diaphragm support perimeter on the side facing the electromagnet, to raise the voltage at which mechanical contact is obtained between the armature rigid with the diaphragm and the electromagnet to beyond the maximum voltage which can be provided by the battery.
This makes the arrangement inapplicable to the circuit configuration of FIG. 1.